1. Field of the Invention
The present invention is directed to a plasma processing apparatus and a method for using the apparatus to process substrates (e.g., semiconductor wafers or liquid crystal display panels), and in particular to the manufacture and use of an asymmetrical focus ring.
2. Discussion of the Background
As is known in the art, a fundamental step in the manufacture of semiconductor devices, such as integrated circuits (ICs), is the process of forming electrical interconnections. The formation of electrical circuits, such as semiconductor transistors, involves a series of steps starting with the formation of a blank silicon wafer. The blank silicon wafer is then processed using successive steps of depositing and etching away various materials to form the proper interconnections and therefore the electrical circuits.
One method of depositing and etching metal layers to and from a silicon wafer includes the use of a plasma reactor system. In semiconductor manufacturing, plasma reactor systems are used to remove material from or deposit material to a work-piece (e.g., semiconductor wafer) in the process of making integrated circuit (IC) devices. A key factor in obtaining the highest yield and overall quality of ICs is the uniformity of the etching and deposition processes.
In a narrow gap, high aspect ratio capacitively coupled plasma reactor, a multipurpose chuck assembly design is often employed that attempts to allow the chuck assembly (i.e. wafer work piece holder) to serve additional purposes other than supporting the wafer. The complexity of the mechanical design of the chuck assembly is such that a non-optimal vacuum system design is required. A vacuum pumping system is used to evacuate the reactor processing region to the low pressures necessary to create a clean environment, to which a specific gas chemistry is introduced, which provides an environment for the generation of plasma. Consequently, due to the complexity of the mechanical design of the chuck assembly, the symmetry of the vacuum system (relative to the wafer) is sacrificed such that the vacuum pump is usually positioned to access the reactor vacuum chamber from the side rather than from the chamber bottom or top.
This type of multi-purpose chuck assembly can become a very cumbersome component of the reactor. In a multi-purpose chuck assembly design, in addition to supporting the wafer, the chuck assembly is typically configured to provide vertical translation in order to reduce the electrode-to-wafer spacing. This spacing control is necessary in order to produce a narrow gap for process conditions and to enlarge the gap for wafer exchange. In addition to the aforementioned capabilities, the chuck typically sustains a radio frequency (RF) energy bias. Moreover, the chuck assembly design further includes components for chuck block cooling, electrostatic clamping, and backside gas flow to improve thermal conduction (between the wafer and the chuck). Consequently, the vacuum design is often a secondary consideration to other various mechanical and electrical component designs.
A reactor chamber that is equipped with a side mount vacuum port is considered an asymmetrical design in a nominally cylindrical system. An inherent drawback associated with an asymmetric design is that it often times produces an asymmetric process. One such asymmetry stemming from an asymmetric vacuum design is the observation of pressure field non-uniformity above the wafer when the chamber is evacuated from the side. That is, a pressure gradient with about 10-20% variation can occur across the wafer being processed. In general, for moderate to high pressures (e.g. P>20 mTorr), a region of low pressure is observed at an azimuthal location adjacent the pump entrance or pumping duct entrance (the pumping duct interfaces the inlet of the pump, e.g. turbo-molecular pump, with the vacuum chamber). In known capacitively-coupled plasma reactors, attempts to solve the problem of an asymmetric chamber flow field introduced by pumping from the side have included the insertion of an orifice plate adjacent to the chuck.
A processing chamber generally includes a single evacuated volume wherein a portion of that volume is proximate the wafer and is hereinafter referred to as the processing region. When an orifice plate is employed, the chamber volume is separated into two regions by the orifice plate. The first region is predominantly occupied by the wafer processing region and the second region, referred to as the pumping volume, is accessed by the vacuum pump. This solution tends to improve the flow-field uniformity in the upper chamber volume by providing sufficient flow resistance through the orifice plate. However, this improvement is achieved at the expense of flow conductance or pumping speed at the processing region. In addition to placing the orifice plate adjacent the chuck assembly, other known designs included locating the orifice plate adjacent other surfaces, e.g. any surface interfacing the processing chamber volume that allows the exhaust of chamber gases.
There are several inherent problems with the known methods that use an orifice plate to control pressure uniformity in a chamber. For example, known orifice plates typically distribute the small openings equally in the azimuthal direction about the orifice plate in the hope that the resultant flow conductance will be azimuthally symmetric through the plate. However, in order to achieve flow-field uniformity, it is necessary to restrict the flow through the orifice plate to the extent that the pressure difference across the orifice is significantly greater than any pressure gradient in the processing or pumping regions. This requires making the holes in the orifice plate small and, hence, paying a penalty in chamber pumping speed at the wafer. This penalty in pumping speed directly results in an adverse effect on throughput.
In addition to the problem of pressure field non-uniformity described above, an additional problem associated with plasma processing systems is the transport of plasma to the pumping duct and pump inlet. In general, the aforementioned orifice plate or a separate pumping duct screen is utilized to attenuate the plasma density prior to reaching the pump inlet. For example, in known systems a pump screen (with generally less than 50% solidity) is placed in the cross-section of the pumping duct. Unfortunately the pumping screen attenuates the plasma and also reduces the pumping speed delivered to the processing region by at least a factor of two. This approach results in at least 50% of the frontal area of the pumping duct cross-section being utilized for recombination surfaces. In conventional designs, there is a one-to-one relationship between the increase in recombination surface area and the decrease in the frontal (flow-through) area.